In continuous ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories. Graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other of the drops strike a drop catcher device. The continuous stream ink jet printing process is described, for example, in U.S. Pat. Nos. 4,255,754; 4,698,123 and 4,751,517, the disclosures of each of which are totally incorporated herein by reference.
In the ink jet printing art, water soluble dyes are desirable for runnability. However, waterfast inks are highly desirable for producing permanent images. Decreasing the solubility of the dye increases the waterfastness, and vice versa. One means for improving waterfastness is to use polymers and resins in inks to enhance adhesion of the dye to substrates. However, presence of resins in inks adversely affects machine runnability due to formation of deposits on orifice plates and charge plate buildup.
Ink jet ink compositions are known and generally contain water soluble dyes. In U.S. Pat. No. 5,017,644, ink jet ink compositions are disclosed which contain linear N-hydroxyethyl substituted polyethyleneimine polymers. Although the '644 patent disclosed some experiments using branched polymers, in addition to the experiments using linear polymers, the '644 patent actually teaches away from using branched polymers. In the '644 patent, the experiments using branched polymers gave results which were inferior to those using linear polymers. Unfortunately, linear polymers is not even commercially available, since the normal polymerization product of ethylenimine has random side branches which upon ethoxylation gives branched N-hydroxyethyl polyethyleneimine.
In U.S. Pat. No. 5,223,028, a hydroxyethylated polyethyleneimine (EPEI) is employed as one of four essential ingredients, Polyethyleneimines decrease the solubility of the dye. Polyethyleneimines degrade the dye, since they reduce the azo-linkage, which is the chemical group responsible for the color of the ink. The use of hydroxyethyl polymer increases the solubility of the dye, relative to the use of the unethoxylated polymer. This, unfortunately, decreases the waterfastness of the ink. Primary amines used in the inks can chemically interact, in an adverse manner, with the dye. Ethoxylation eliminates the presence of primary amino groups in the polymer, and the '028 patent requires ethoxylation of at least 95%. Unfortunately, when ethoxylation approaches 95%, the dye becomes more soluble, which can decrease the desired waterfastness of the ink. With the greater solubility, the '028 patent teaches lowering the pH by adding an ammonium salt. The salt is a required ingredient in the ink of the '028 patent. In addition, the invention of the '028 patent is no longer a pure compound. Reaching the required ethoxylation of at least 95% increases the potential of a side reaction between ethylene oxide and the hydroxyethyl chains of the polymer, forming polyglycol ether by-products. This would further increase the solubility of the dye, as previously mentioned.
It is seen then that there is a need for improved waterfast dye for use in ink jet recording equipment.